Communication involving airborne transceivers is subject to radio propagation which is often well described by a radio propagation channel model with two propagation paths, herein referred to as a two-path channel model. This two-path channel model has one direct path from transmitter to receiver, and one in-direct path from transmitter to receiver, via at least one reflector like a surface plane or similar.
Communication involving airborne transceivers also often take place over long distances. Examples include communication between a surface based unit and an airborne unit beyond the horizon, and also communication between two airborne units where the air-to-air communication link can extend to distances such that the line of sight between communicating nodes touches or almost touches the horizon.
This type of long distance radio link, characterized by the two-path channel model, is subject to challenging multi-path fading conditions which complicate communication between network nodes. To further complicate matters, strict requirements on, e.g., transmission capacity in terms of information bits per second and tight delay communication deadlines must often be met.
The challenges related to the type of long distance communication described above are mainly related to the distance dependency of the multi-path fading as predicted by the two-path channel model. A problem related to said distance dependency is that the fading condition, i.e., the propagation path gain, varies slowly or very slowly as the propagation path distance varies, i.e., the fading process is comparably slow compared to short distance communication links. The effect is especially pronounced at lower frequency bands, and becomes somewhat less pronounced at very high frequencies. This means that a large displacement of communication transceivers is needed in order to achieve a significant difference in communication conditions, e.g., in received signal power. Hence, two communicating nodes may experience poor propagation conditions during extended periods of time, even if the relative velocity and direction of motion of the communicating nodes are such that the propagation link distance changes by several kilometers during a transmission time interval of interest. Note that the propagation path distance is determined by the relative coordinates of the communication nodes and the at least one reflector, including latitude, longitude, and altitude.
These propagation conditions are different compared to the propagation conditions experienced during shorter distance communication, where fading is often much faster, i.e., where there is a much stronger distance dependency, and it is unlikely that two communicating nodes comprising an airborne node will experience poor multi-path conditions during extended periods of time, such as during an entire transmission time interval of interest.
Another propagation phenomenon which can occur during long distance communication is so-called ducting, where the propagation path of a transmitted signal is directed away from a straight line of propagation by different layers in the transmission medium. The propagation path then becomes bent, or curved.
Of course, more than two propagation paths can be included in the channel model to better model certain propagation conditions. Such additional paths may for example arise due to diffraction or ducting phenomena. However, the fundamental problems discussed herein remain essentially the same. Thus only the two-path channel model will be discussed herein.
Previously proposed solutions to combat multi-path fading, ducting, and other related propagation phenomena include equipping each individual node with more than one antenna, or using spread spectrum techniques which utilize a wide bandwidth for communication in the hope that some parts of the utilized spectrum will be free from adverse fading effects.
However, due to the slow fading process mentioned above, i.e., the slow changes in propagation path gain with changing link distance in long distance communication links, previously proposed solutions to combat multi-path fading are often ineffective.